1. Field of the Invention
This invention concerns a metal-coating composition containing acrylic polyol(s), polyester polyol(s) and a crosslinking agent.
2. State of the Art
The coating composition of this invention is useful primarily on metallic-substrates, particularly on vehicles such as automobiles and trucks. Such coating will protect the substrate and provide an attractive aesthetic finish. A typical vehicle has several coating layers. The substrate is typically first coated with an inorganic rust-proofing zinc or iron phosphate layer over which is applied an electrocoated primer or repair primer. Optionally, a primer surfacer can be employed for better appearance and improved adhesion. A pigmented basecoat or colorcoat is next applied over the primer. A typical basecoat or colorcoat may contain metallic flakes to provide a metallic finish. To protect and preserve the aesthetic qualities of the color finish, a clear (unpigmented) topcoat is often applied over the pigmented basecoat to protect the basecoat even on prolonged weathering. Clearcoats are predominantly based on a technology where the binders are hydroxy functional acrylics crosslinked with alkoxylated melamine formaldehyde adducts. The coatings are typically baked at .+-.130.degree. C. after application wet-on-wet on a basecoat. Because of severe field complaints, the automobile coating industry is asking for improved scratch resistance of the clearcoats.
The composition of this invention has been found to provide particularly desirable scratch-resistance to the metallic substrate coating art. This improvement is in addition to attributes of good weatherability, chemical-resistance, water-resistance, etch-resistance and mechanical characteristics. This improved package of properties derives from the interrelationship of the selected acrylic and polyester polyols and the crosslinker selected for use therewith. Most particularly, the improved properties are derived from the dimer acid/alcohol component of the polyester polyol as will be described in more detail hereafter.